Measurement of the neutral weak form factors of the proton

Abstract

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [͗u lab ͘ 12.3 ± and ͗Q 2 ͘ 0.48 ͑GeV͞c͒ 2 ] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G s E . The result, A 214.5 6 2.2 ppm, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies G s E 1 0.39G s M 0.023 6 0.034͑stat͒ 6 0.022͑syst͒ 6 0.026͑dG n E ͒, where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor. [S0031-9007(98) The proton, which is believed to be a state of three quarks bound by the strong force of quantum chromodynamics (QCD), is a complex object when probed at intermediate energies. In order to develop a useful description, one must first establish all of the relevant degrees of freedom. Recent theoretical and experimental investigations have indicated that strangeness might play an important role [1,2]. For example, do ss pairs contribute to the charge radius or magnetic moment of the proton? It is quite possible, since the mass of the strange quark is comparable to the proton mass and the scale of the strong interaction. On the other hand, the empirically successful 1096 0031-9007͞99͞82(6)͞1096(5)$15.00

First Determination of the Weak Charge of the Proton

“…The uncertainty of the asymmetry reported here is less than that of previous parity-violating electron scattering (PVES) experiments [10][11][12][13][14][15][16][17][18][19][20][21] directed at obtaining hadronic axial and strange form-factor information [22]. The theoretical interpretability of the Q weak measurement is very clean as it relies primarily on those previous PVES data instead of theoretical calculations to account for residual hadronic structure effects, which are significantly suppressed at the kinematics of this experiment.…”

“…[12,49]. R Det ¼ 0:987 AE 0:007 accounts for the measured light variation and nonuniform Q 2 distribution across the detector bars.…”

“…Following the procedure outlined in [6,22], a global fit of asymmetries measured in PVES [10][11][12][13][14][15][16][17][18][19][20][21] on hydrogen, deuterium, and 4 He targets was used to extract Q p W from Eq. (4).…”

Lepton mass effects for beam-normal single-spin asymmetry in elastic muon-proton scattering

“…For this reason, one finds the beam-normal SSA to be of order 10 −6 −10 −5 for scattering of a polarized electron beam of GeV energy. An asymmetry of a similar order can be observed in parity-violating electron-nucleon scattering experiments [41][42][43][44][45][46][47][48][49]. Measurements of the parity violating asymmetry (A PV ) involve a longitudinally polarized electron beam and have been of significant interest for decades, as they provide a high-precision test of the Standard Model and enable one to extract a strange quarks contribution to electromagnetic form factors of the nucleon.…”

Recent results from Jefferson Lab

“…Hadronic models describe the data over the entire range in momentum transfer. Bates [4] VEPP−3 [5] Bates [2] NIKHEF [6] Jefferson Lab NRIA [8] NRIA + MEC [8] CIA [10] pQCD [16] t 20 t 21 t 22 Fig. 13.…”